Extrusion dies are used in manufacturing processes to make a variety of goods. Some dies, for example, are used to form thin films, bars or other elongated shapes of plastic material. It is known in the art that many advantages are achieved by the production of multi-layer film constructions of thin films (e.g., using thermoplastics) as this construction enables a combination of properties not available in the unlayered structure. Originally such products were prepared principally by laminating separately formed films or sheets together by adhesives, heat or pressure. Techniques have been developed, however, for melt laminating which involves joining two or more diverse materials (e.g., thermoplastic materials) from separate molten layers under pressure within a die to emerge as a single laminated material. Such processes make use of the laminar flow principle which enables the two or more molten layers under proper operating conditions to join in a common flow channel without intermixing at the contacting interfaces. These multiple layer extrusion systems have come into prominent use as a convenient way to provide for the formation of multiple layers of similar or dissimilar materials.
Various extruded film devices (e.g., dies) have been manufactured to extrude the multiple layer films. One general configuration of device utilized a first die section which combined the various layers of materials. The combined materials were then flattened and extruded through a second die section. An example of this type of device is illustrated by U.S. Pat. No. 5,316,703, incorporated by reference herein in its entirety. This type of device was limited in effectiveness because of thin film manufacturing which requires the multi-layer sheet (or web) to have uniform thickness across the width of the extruded sheet. In particular, if there are great differences in viscosity, temperature, and flow rate, between melted resins which form the resin layers, it can be difficult to obtain multi-layer sheets with the uniform thickness of each layer that is desirable for multi-layer extruded sheets.
Multi-manifold die systems are designed with an individual flow channel/manifold for each layer and normally the layers are brought into contact just before the exit of the die. Because the layers are joined only near the final exit slot, materials with somewhat diverse rheological properties can be processed. The individual layers can be formed at the desired thickness before combining with the remaining layers and adjustments of the flow speed for each individual layer can be effected to maintain uniformity of flow between the various layers. This is necessary, since any tendency towards differences between flow at the junction point between layers can cause non-uniformity in the product.
Additionally, slide coating and slot coating apparatuses have been developed which allows multilayer coating of fluids while they flow down an inclined plane. These types of devices are disclosed in various textbooks, for example, Hens, Jules and VanAbbenyer, Willy “Slide Coating” in: Kistler and Schweizer, Liquid Film Coating (London, Chapman & Itall, 1997), pp. 427-462. Durst, Franz and Wagner, Hans-Günter “Slot Coating” in: Kistler and Schweizer, Liquid Film Coating (London, Chapman & Itall, 1997), pp. 401-426. The text of these references are incorporated by reference in their entirety herein. These methods of coating require fluids having low viscosity at room temperature which allow the fluids to freely flow. However, these apparatuses do not work for higher viscosity materials, such as thermoplastics, which are fluid only at higher temperatures (e.g., approximately 150° C.). Even at these temperatures, thermoplastics often have high viscosities that would prevent the use of gravity to drive their flow.
Patents which disclose devices for forming multi-layer laminates include U.S. Pat. No. 4,152,387 (Cloeren), U.S. Pat. No. 6,203,742 (Kegasawa et al.) and World Intellectual Property Organization International Publication number WO 01/08866 A1 (Norquist, et al), all of which are incorporated by reference herein.
These devices are limited, however, in the number of layers which can be provided in the extrudate. Attempts to extrude many layers results in dies which are cumbersome and problematic to machine. It is desirable, therefore, to create an extrusion device which allows for a large number of layers to be created in a multi-layer sheet, while simultaneously providing small thickness tolerances for each layer, across the entire width of the sheet.